Get 6 small teams of fans that represent each previous game (forget civ7 exists). If you can get all of these 6 groups to like civ8, then I think you are set up for success
One thing that I've become aware of through the discussions here, though, is that there are now significant dichotomies that have developed within the fanbase. And they have developed in part because of individual preferences but also in part because previous numbers of the game did one thing or another in a particular way, and fans of that game came to like it
and regard it as definitive of the franchise. I'm trying to compile as complete a list as I can, because I think it would help illustrate what any new set of devs are up against when they undertake to design a new number of the franchise. So feel free to add any that I haven't thought of, but here is my list:
Should it be a computer game or a board-game on a computer? (I don't quite understand this one, because 3 supposedly falls into the computer-game era of the franchise, but it feels like a board-game to me; turns and a map make anything feel like a board-game to me, so I obviously don't understand what people are driving at when they say 1-4 were computer games. But I do know it's one of the dichotomies that splits fans).
Should it involve entirely high-level empire-management
strategy or can it include the
tactical-level dimension that came in with 1UPT?
Should it maximize or minimize micro-management? (for my part, I love micro-management; I love feeling I'm getting the tiniest edge by some little thing I do with my tile assignments or tile development; these tiny edges add up to my advantage in the game, so I think of them as
how I am competing in the game. But I also totally understand that there would be people who find micro-management tedious and want "optimization" instead).
Should it be built primarily as a multi-player game or primarily as a single-player game, and then the other derived out of that starting basis?
Should it favor wide or tall? How many cities should a normal/winning empire have in them? (Civ 5 made playing wide difficult, but then, does the 4-city empire that is optimal there really end up feeling like an
empire? On the other hand, if your empire covers dozens of cities across the whole globe, does managing those cities become tedious?)
Should the AI civs play to win the game or just be part of the background environment in which you build your own empire?
Should the game prioritize the meeting of victory conditions, or should it most fundamentally allow sandbox-style play?
Should the game resist snowballing, or is snowballing just a negative term for the
very thing you should be trying to do in the game: min-max so as to build up an advantage on all of the other civs?
How significant should the changes be from the past iteration to the next? Should the game-play mechanics work essentially the same way, so that all of your skills in # carry over into #+1? Or should a new # have fresh mechanics from the ground up? ("I don't want no #.2").
So I'm not sure that, at this point, you could get your 6 focus groups to agree on what would make a good direction for 8. (Plus, people who like elements of 7 would feel resentful at being excluded and would boycott your stinkin' game).